The detecting capacity is in inverse proportion to tunnel size and distance from coil.
Generally, magnetic field from metal detector is generated by AC voltage(from several tens kHz to hundreds kHz) Magnetic intensity is in proportion to electric intensity and in inverse proportion to distance from coils. The bigger tunnel size, the lower magnetic inductance. Magnetic force is decreased because of long distance from coil .

Metal detector recognizes water or salt with a metal. So it have detector make a mistake.
Generally many kinds of products contain water & salt and there are much water and salt in production site. Detecting sensitivity is reduced depending on contained quantity occasionally. The sensitivity has to be adjusted to avoid recognizing water or salt with a metal. So proper product character value is adjusted, considering detecting sensitivity.

High frequency from invertors affects head of detector and has detector make a mistake.
There are a lot of invertor's motors in production site. These affects detecting capacity badly. Every metal detector detects metal, using high frequency. Invertor also controls motor speed, using high frequency. In any case, the deteching capacity is reduced about 4~50%. If signal line of invertor's motor is separated from detector, it is very effective in this case.

Electric impact from frictional electricity affects PCB circuit and make detector make a mistake or be out of order forever.
There is a lot of frictional electricity in our life and production spot. This causes a fire in paper manufacturer or flourmill company. Occasionally this also causes detector to be out of order forever. So we have to consider a counterplan before input the product.